Chromosomal instability is a hallmark of human cancers, and tumors derived from individuals with Li-Fraumeni syndrome (LFS) often contain a multitude of aberrant chromosomes. Increasing evidence from human and mouse model systems suggests that proper maintenance of telomeres is important for chromosomal integrity and genome stability. Telomere dysfunction initiates the formation of the breakage-fusion-bridge cycle, leading to genome-wide alterations in gene dosage. In the setting of mutant p53, these diverse genetic alterations are hypothesized to fuel cancer initiation and progression. To test the hypothesis that the most common gain of function mutation in LFS (the p53R172H mutation corresponding to the p53R175H hotspot in human cancers) in the setting of telomere dysfunction promotes chromosomal instability and accelerates the onset of tumorigenesis in vivo, we have generated mice with normal or dysfunctional telomeres heterozygous or homozygous for the p53R172H mutation. We will compare the latency of tumor development, tumor spectrum, types of chromosomal aberrations observed, and telomere length of primary and established tumor cell lines between these four mouse cohorts. We will determine the presence of micro-satellite instability (MSI) in tumors derived from telomerase null mice to determine whether MSI is a hallmark of tumors where telomeres have been restored by a mechanism that involves alternative lengthening of telomeres. The clear generational impact of telomere dysfunction in the telomerase knockout mouse, coupled with exciting new data suggesting that the severity of the disease status in patients with dyskeratosis congenita correlates with progressive telomere shortening in later generations, implies that inheritance of short telomeres may be responsible for disease anticipation. We hypothesize that telomere dysfunction may be the basis for the earlier onset of cancers observed in descendents of affected LFS patients. We will test this hypothesis by isolating peripheral blood samples from parents and children of age and sex-matched controls and LFS families with different types of p53 mutations and determine the telomere length in these samples using flow-FISH, quantitative-FISH and terminal restriction fragment (TRF) length analyses. We anticipate that inheritance of short telomeres will lead to presentation of cancers at a younger age. Relevance: the proposed research has direct relevance to public health, since telomere length shortening may be a biomarker for overall genomic instability in patients with LFS. We believe that telomere dysfunction will prognosticate the early onset of tumors in patients with LFS. If this is true, then early diagnostic testing for average telomere lengths in LFS families will be extremely important. For example, LFS patients with very short telomeres may warrant more frequent monitoring for elevated cancer risk. In addition, emerging studies from other human diseases such as Dyskeratosis Congenita and Aplastic Anemia indicate that it is likely that telomere shortening may prognosticate elevated disease risk in the general population. It is therefore possible that telomere length determination may become standard medical practice in the near future.